Conventionally, there have been proposed various types of buckle devices provided with a detection mechanism to detect the insertion of a tongue plate into a buckle main body.
For instance, a conventional buckle device is provided with a cover which is approximately rectangular tube-shaped, and open-ended at both lengthwise ends. An attachment piece is inserted from one side thereof, and then the cover is mounted on the buckle main body. Then wing portions of a switch is bent outward to widen the space therebetween, and placed outside the attachment piece. While maintaining this state, the switch is inserted inside the cover through a predetermined clearance formed on the other side of the cover. Thereby, such a buckle device is configured to be able to detect the insertion of the tongue plate into the buckle main body (see, for instance, Japanese Laid-open Patent Application Publication No. 2000-4909).